1. Field of the Invention
The present invention relates to a distance measurement apparatus for a camera having an auto-focus mechanism for driving a photographing lens on the basis of an output from a distance measurement means, and a distance measurement method.
2. Description of the Related Art
A camera having an auto-focus mechanism has a long release time lag from when a release button of the camera is depressed until an exposure operation is started. For this reason, when an object moving in the optical axis direction of the camera is to be photographed, a defocused state of the photographing lens occurs.
As a means for solving the above-mentioned problem, Published Unexamined Japanese Patent Application Nos. 63-118133 and 63-159817 disclose an apparatus, which detects the moving speed of an object to predict the position of an object at the beginning of exposure, and drives the photographing lens according to the predicted position.
However, in this prior art, complicated arithmetic operations are required for predicting object movement, and the release time lag is increased, thus missing a shutter chance. Furthermore, the arithmetic operations are performed under a condition that the moving speed of an object is ideally and precisely obtained, and do not consider the influence caused by an error of the moving speed. As a result, the following problems are posed.
More specifically, in a distance measurement apparatus, e.g., in an active type triangulation apparatus, which provides a basis for obtaining the moving speed of an object, as shown in FIG. 10, its output varies and has a given indefinite width W in each distance measurement. The indefinite width W has the following tendency. That is, the indefinite width W is small for a near distance at which the light amount is large, but becomes large for a far distance at which the light amount is small.
For example, when an object moves from 1/l.sub.O to 1/l.sub.1, the distance measurement value may change from a to d, b to e, or c to f. The change from a to d corresponds to an extreme case. In this case, although an object is moving toward a camera, a moving speed as if it were moving away from the camera is obtained.
When correction is made based on the change from c to f, this results in overcorrection and a defocused state.
Furthermore, when the time required for obtaining the moving amount of an object is assumed to be, e.g., 50 msec, and the release time lag is assumed to be 400 msec, an object distance at the exposure start time after an eIapse of 400 msec is predicted on the basis of the moving amount of the object during the 50-msec period. As a result, a moving amount error if any is
amplified to 400/50=8 times, and the possibility of a defocused state is further increased.
However, when the number of times of light projection in each distance measurement is increased or the time for measuring the moving amount of an object is prolonged to reduce the error, the time lag is further increased, and a shutter chance itself is missed.